1. Field of the Invention
The present invention relates to an improved armature of an electric rotating machine and more particularly to an improved armature of a D.C. rotating machine having a commutator.
2. Description of the Prior Art
In an armature of a conventional commutator machine such as a DC machine, provision is made to include a large number of linear axial slots formed on the surface of an armature core, and an armature winding composed of upper and lower coils connected to commutator segments and mounted in the slots. Each of the upper and lower coils normally includes about one through five coil sides (i.e. conductor elements disposed in each slot) connected to different commutator segments respectively.
FIG. 1 shows schematically the armature of the DC machine where each of the upper and lower coils is arranged with a single coil side. In this figure, 10 denotes a tooth of an armature core, 11 denotes an axial slot, 12 denotes an armature winding, 12a.sub.1 and 12a.sub.2 denote upper coil sides of the armature windings, 12b.sub.1 and 12b.sub.2 denote lower coil sides of the armature windings, 13 denotes a commutator, 13a-13c denote commutator segments, and 15 denotes a brush which is contacted with the commutator 13.
When the brush 15 has reached position A, the commutation of the upper coil side 12a.sub.1 and the lower coil side 12b.sub.1 is terminated. The rate of current change di/dt at this time produces slot leakage flux .phi..sub.l and hence a reactance voltage er expressed by ##EQU1## where L denotes the inductance of the commutating coils. If this reactance voltage is higher than the sparking threshold voltage, spark is generated at an exit side of the brush 15 and this spark adversely affects the performance of the DC machine. In the case of FIG. 1 where each of the upper and lower coils is composed of a single coil side, when the brush has reached the position B with a given rotation of the commutator 13 in the direction of the arrow R, the reactance voltage induced in the upper coil side 12a.sub.2 and the lower coil side 12b.sub.2 is the same as that induced in the upper coil side 12a.sub.1 and the lower coil 12b.sub.1. Accordingly, the reactance voltages are substantially balanced among the commutating coils, so that the machine of this type has no problem on commutation.
FIG. 2 shows a case where each of the upper and lower coils are composed of two coil sides. When the brush 15 has reached the position A, the commutation of the upper coil side 12a.sub.1 and the lower coil side 12b.sub.1 is terminated, while when the brush 15 has reached the position B, the commutation of the upper coil side 12c and the lower coil side 12d is terminated. In this case, there exists a difference in the reactance voltage er induced by the commutation between the positions A and B. More specifically, when the brush 15 is positioned at A, the variation of the slot leakage flux .phi..sub.l produced by the commutating current through the upper coil side 12a.sub.1 and the lower coil side 12b.sub.1 is suppressed because the upper coil side 12c and the lower coil side 12d mounted in the same slots as those in which the coils 12a.sub.1 and 12b.sub.2 are mounted, respectively, are short-circuited by the brush, so that the reactance voltage induced is lower. On the other hand, when the brush 15 is positioned at B, the variation of the slot leakage flux .phi..sub.l produced by the commutating current through the upper coil side 12c and the lower coil side 12d is not suppressed because no short-circuit is established through the coil sides mounted in the same slots, so that the reactance voltage induced is higher. Accordingly, this prior art machine has drawbacks in that the commutating performance is unstable and affected by the variation of the reactance voltage of the upper coil side 12c and the lower coil side 12d. Particularly, when the number of the coil sides constituting each coil becomes larger, this trend becomes greater so that the width of sparkless zone representing the commutation performance becomes narrower.